As is well known, a magnet coil can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance of the magnet coil to negligible levels, such that when a power source is initially connected to the coil for a period of time to introduce a current flow through the coil, the current will continue to flow through the coil due to the negligible coil resistance even after power is removed, thereby maintaining a strong magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter "MRI").
MRI equipment is relatively expensive such that a single installation is frequently shared by a plurality of medical facilities or medical practitioners. While there are regional fixed MRI locations, there are also a plurality of mobile MRI installations installed in tractor trailers which are periodically moved to a plurality of geographically spaced locations to serve such a plurality of locations, typically smaller communities that cannot afford to maintain an MRI installation.
There are a number of problems encountered by a mobile MRI. A mobile MRI is frequently operated in a parking lot adjacent to a hospital or medical facility and is a self-contained imaging room including the MRI equipment, space for the operator along with related controls and accessories plus space for the ingress and egress of patients. However, because of the extremely strong magnetic fields generated by MRI equipment, there are applicable regulations limiting the area covered by the strong magnetic field, requiring the spatial confinement or restriction of the stray magnetic field in the region surrounding the MRI equipment. In a hospital imaging room this stray magnetic field confinement is frequently accomplished by large and extremely heavy shields of ferromagnetic material placed, for example, in the walls and ceiling of the imaging room and/or around the MRI magnet. However, problems in maintaining the necessary field homogeniety in the MRI imaging bore must be considered and resolved in any shielding of an MRI superconducting magnet.
Mobile MRI installations must also meet additional applicable federal and local regulations such as Department of Transportation regulations limiting the weight of vehicles on roads and bridges. Other reasons, including those of economy, have resulted in the inability to effectively shield a high field mobile MRI superconducting magnet installation in the order of field strength 1.0T (Tesla) or greater sufficiently to transport the MRI equipment "at field" or in the superconducting state.
The regulations regarding the magnet's field confinement require that the magnetic field strength not exceed five gauss 6 inches beyond the walls of an installation if the general public is not restricted from the area. One reason for such a regulation is that of inadvertent exposure to a strong magnetic field by a person wearing an electronic heart pacer regulating the heart action of the wearer can cause misfunction or failure of the pacer, and possible death of the person. Also, strong magnetic fields can adversely affect the operation of other electronic equipment in the vicinity including that within adjacent vehicles such as police or other emergency vehicles.
As a result, it is common practice to erect a temporary barricade a safe distance around a parked and operating the high field mobile MRI superconducting magnet unit to prevent persons or equipment coming within the region where the magnetic field is in excess of five gauss. However, when moving such mobile MRI unit from one location to another, superconducting magnet operation must be discontinued because of the inability to control the passage or presence of vehicles and persons on the highways and elsewhere, such as at rest stops, coming within the strong magnetic fields which extend beyond 6 inches from the trailer walls.
However, such mobile MRI units discontinue superconducting magnet operation during the periods of transporting the MRI unit from one site to the next even though the next site may be at the next town or relatively close to the previous site. This leads to considerable additional expense, time (including equipment down time) in discontinuing the superconducting magnet operation at one site, and in ramping up or reestablishing such operation at the next site. As a result, there has been a long felt need for a practical, yet lightweight, economical method of effectively shielding high field mobile MRI units to enable their transportation between sites while at field or in superconducting operation.